1. Technical Field of the Invention
The present invention relates to switching the frequency of a clock signal.
2. Description of Related Art
It is a requirement in certain applications, such as with communications systems that support different data rates, for the frequency of a clock signal to be switched between different frequency values on an instantaneous (or nearly instantaneous) basis. This requirement arises from the need to ensure that communicated data is not lost while the communications system supports and switches between different data rates.
Clock generation circuitry of the Phase Lock Loop (PLL) type is commonly used to generate clock signals. These PLL-type clocks can be controlled (or programmed) to switch the clock signal frequency among and between two or more clock frequencies. However, there is generally a need for a lock time to be associated with each frequency change. Instantaneous frequency change of the clock signal is not possible due to the delay introduced by the lock time.
A need exists in the art for a PLL-type clock generation circuit having a controllable frequency that can be switched on-the-fly between at least two frequencies without requiring a lock time. Preferably, this PLL-type clock generation circuit will maintain frequency lock, phase lock and clock de-skew as the output clock makes frequency changes.